Electric ship propulsion



H. M. HOBART. ELECTRIC SHIP PROPULSION. APPLICATION FILED MAY I9. 1916.

1,331,940. v Pa tented Feb. 24,1920.

Fig. I.

I Inventor": I

. I HISO T or'r eg.

UNITED sTATEs PATENT OFFICE.

HENRY I. HOBART, OF SCHENECTADY, NEW YORK, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

ELECTRIC PROPULSION.

To all whom it may concern:

Be it known that I, HENRY M. HOBART, a citizen of the United States, residing at Schenectady, in the county of Schenectady, State of New York, have invented certain new and useful Improvements in Electric -Ship Propulsion, of which the following is a specification.'

My invention relates to electric ship propulsion, and particularl to systems of ship propulsion in which the propellers are driven by alternating current electric motors. The object of the invention is'broadl y to provide an improved system of electric ship propulsion. More particularly, the object of the invention is to provide an electric propelling-system for ships which is not only effective for propellmg and maneuvering the ship, but at the same time is ecodrives the ships propeller. A polyphase induction motor is preferably associated with the synchronous motor for starting'or for obtaining an increased motor torque for certain conditions of navigation;-, A high speed synchronous condenser driving a continuous current generator is also preferably provided for theexcitations of the induction generator and propeller-driving synchronous motor. aspect of my invention, the combination induction-synchronous motor drive for the propeller is so developed as to enable, the use of a high speed synchronous generator instead of an induction generator, since a large part of the excitation of the synchronous generator can be furnished by over-exciting the synchronous motor.

- It is well known that'the higher the/speed employed in steam turbines, the lower is 1 KW steam turbine could, with advantage, be v their cost and the better is their economy. With our presentfknowledge of the, design of such machines and with the materials available for their constructioma 12,000

designed for a speed of 3600 rpm. or more.

But the present limit encountered in the'de- Specification of Letters Patent.

In accordance with another Patented Feb. 24, 1920.

Application filed Kay 19, 1916. Serial No. 98,548.

sign and construction of satisfactory synchronous generators is only about one-half big a speed as 3600 rpm. But in contrast with the limitations to the design of a synchronous generator, it maybestated that an induction generator with a squirrel-cage rotor can be designed in any size for as high a speed as would be appropriate for the steam turbine to which it is coupled. Indeed, the higher the speed of an induction as reat (namely, some 5000 KW) for so generator, the better is its power factor, and,

consequently, thelower is the amount of excitation which need be provided for it from the associated synchronous apparatus, such as a synchronous condenser or synchronousmotor electrically connected thereto. In

ship propulsion there is no necessity for adhering to the standard 'periodicities employed in land installations, since each ship constitutes a self-contained design. Consequently, a higher speed than 3600 rpm. (which'corresponds to 60 cycles with a twopole machine) may be employed. But.

3600 rpm. is a fairly high speed, and since it is a familiarvalue to electrical engineers, it will be employed in the examples 'by which this invention will be described.

Qmust have a very low speed. For large ships, speeds'of the low order of some 60 to 200 rpm. are usually desirable in the interests 'of obtaining good propeller efficiency .and cod maneuverlng properties. One of the dlfliculties with an induction motor drive for a ships propeller is that, since for good efficiency the propeller must be designed for a relatively low speed, the induction motor has td-have a very large number of poles.

This requires that the motor be designed w1th a very large diameter, and, unless the motor is for very large power, it will be of anundesirably short length, and as a consequence its cost will be great and its efficiency and power factor poor. These bad features are accentuated by the desirability of employing a'very high-speed turbo-generator so as to have low fuel consumption. To obtam thls high-speed turbo-generator it bewere it not that the higher the periodieity "comes necessary to adopt a rather high periodicity and 60 cycles is often desirable.

erator set must be desi I 1 than 3600 rpm. and thlS lIlVOl-YGS' higher fuel.

1 fuel consumption; so undesirable, but there tion motor for a given low s consumption for driving the ship. It is not alone the cost of the fuel which makes high is the further circumstance in the case of a ship that high fuel consumption means decreased cargo capacity or decreased length ofvvoyage between coalin'g stations. Every time a, ship must put into a port for fuel heavy harbor charges are incurred. Furthermore, the greater the length of the voy-f age which a ship can make without refueling the greater is the. likelihood of being able to put in for fuel .at a ort where the price of fuel is low.- Font ese reasons I employ in mypresent systemof propulsion synchronous motors for'driving the ships propellers, because synchronous motors, While still faulty in certain of-the respects mentioned, are preferable to induction mo tors "at low speeds by reason of the smaller iron losses and. consequently better ower factor, and especially at the high periodicit'ie's corresponding to economical steam turbine generating-sets.

For starting and for conditions of navigation requiring a relatively high motor torque I prefer to provide an induction motor in combination with the propeller-driving synchronous motor. In this connection, I have, furthermore, provided an induction-synchronous motor combination which enables the use of a high-speed bi-polar synchronous generator of large capacity in a systenr of propulsion. One of the particular disadvantages of a system of electric ship propulsion having induction motors for driving the propellers isthat the bad power factor of the induction motor makes it necessary that the generator must have a relativel great capacity in KVA, as com ared with its output in KW. By the use inductionsynchronous motor units in accordance with my present invention the power factor of the system can be adjusted to unity,in fact the system can even be operated with a leadmg power factor, by sufficiently over-ex'citing the synchronous motors. This arrangeto the simpler ment permitsthe field of a synchronous turbo-generatorto be relieved from furnishing a very considerable part of the total excitation which would otherwise be required of it, andso facilitate the'design of a 3600 rpm., bi-polar, synchronous generator of the desired capacity, that the use of, Such'generatorsbecomes pos sible,'when desired; v The foregoino discussion relates broadly present invention. I have further provided certain advantageous modifications in and additions .to the system as hereinhefore ;rent stator winding 18..

forms of the system of my broadly defined which will be fully described hereinafter. which I believe to be patentably characteristic of the invention are definitely pointed out in the claims appended hereto. The arrangement of the various apparatus in the system of my present invention and the mode of operation of the system will be better understood from the followin description taken in conjunction with t e accompanying drawings, in which;

Figure 1 diagrammatically illustrates one form of my invention which has been hereinbefore particularly referred to; Figs. 2, 3 and 4 diagrammatically represent three combinations of an induction motor associated with the'propeller-driving synchronous motor of the system of Fig. 1 for the purpose of obtaining an increased motor torque for :reversal and maneuvering in general; Fig.

5 dia rammatically represents what I now consi er the preferred combination of propeller-driving. induction and synchronous motors for use in the system of Fig. l; and Figs. 6 and 7 are detall views showing the construction, of the rotor of the induction motor unit of the propeller-driving combination of Fig. 5. I

.Referring first to Fig. 1 of the drawings there is diagrammatically represented a high speed elastic fluid turbine 10 directly coupled to an induction generator. The induction generator comp-rises a rotor 11,

' carrying a squirrel cage winding 12 of the usual type, and a three phase stator winding 13. A synchronous condenser. or synchronous motor 14 provides the excitation for the induction generator, while a continuous current generator 15 driven by'the synchronous motor 14 through speed reduct1on gearing 16 provides the direct current excitation for the synchronous motor 14 and the propeller-driving synchronous motor 17.

The novel features The rotor of thesynchronous motor 17 is:

mounted-on a shaft 33 carryin a propeller 34. In Fig. 1 ofthe drawings t e propeller drivin synchronous motor 17 is represented as ving-a three phase alternating cur- A double throw switch 19 operates when thrown to its lower position, as viewed in the drawings, to connect the winding 18 to bus bars or leads 21 electrically communicating with the three phase stator. winding 13 of the induction generator. A disconnecting switch 22 1s 1ncluded in the bus bars 21 and a reversmg switch 15 is provided for reversing the 'direction of rotation of the propeller-driving synchronous-motor 17. The continuous current generator is adapted to be connected .by a switch 23to the direct current mains 24:. The field winding 25 on the rotor of the synchronous motor 17 isconnected to the direct current mains 24 by-means of a switch 26, and similarly the field winding 27 of the synchronous condenser is connected to the .direct current mains 24 by a switch 28.

particular advantages of my present system I will cite a specific equipment for which employed for purposes of 1i hting" power and heating, throughout the sll the system has been particularly designed. In this equipment the speed of the two-pole turbo-generator is 3600 rpm. and its rating. is 10,000 KW. The propeller-driving synchronous motor 17 has poles and its speed is about rpm. This data correspond to electric energy of a periodicity of about 60 cycles. per second. The synchronous condenser of the exciter set will require to have a capacity of some 1500 to 2000 KVA, if it supplies the entire excitation (for the induction generator, but ordinarily it will be desirable to supply a considerable portion of the generators excitation by over-exciting the field of the propeller-driving synchronous motor, in which case the synchronous condenser may be ofsmaller capacity. The synchronous condenser is preferably a bipolar machine and-its speed is hence about 3600' rpm. The synchronous condenser drives the continuous current generator 15 at a'speed of about 360 rpm. through the 10 to 1 reduction gearing 16. The rating of the continuous current enerator 15 need be only about KW, i its only dut is to excite the synchronous apparatus. ut the generator 15 may with advantage also be.

ip, and it will thenbe of several times this rating.

The propeller-driving synchronous motor. 17 maybe provided with a suitable squirrel cage winding35 near the. periphery of its rotor to endow it with good torque at starting. Preferably, however, an auxiliary induction motor 20 is mounted onthe same shaftas the synchronous motor. This induction motor may be of the high resistance squirrel cage type or may have. a polar sec-' ondary winding 31 connected to an external resistance. 32. The induction motor 20 is intended. to be used only for a short time, as whn starting, reversing or maneuvering the ship. It is designed with great heat storage capacity so that its temperature will,not rise unduly during the shorttime's it is in service. In the equipment I have just described the induction motor may have only 40 poles. Then,.although its synchronousspeed on a. 60 cycle circuit is rpm., the load upon it when it is in circuit will always be so great that, since its secondary circuit has high resistance, it can never .run above some 90 rpm., corresponding to the normal propeller speed .and to some 50 per cent. rotor slip of the induction motor. In starting or maneuvering the double-throw switch 19 is thrown to its upper position, as viewed in Fig. 1, whereupon the primary winding of the induction motor is connected in series relation with the alternating current winding 18 of the synchronous motor 17. The s nchronous motor will thus start up with t e induction motor, and since the slip of the latter will usually be still greater than 50 per cent., previously mentioned, the work will be taken over by the main synchronous motor whenever the speed is above say 70 rpm., and hence the synchronous motor will be designed to have high torque above twothirds speed. It will be understood that as desired or as the equipment in hand may require either the squirrel cage winding 35 or the auxiliary induction motor 20, or both,-

may be employed for obtaining the necessary torque at starting.

The induction generator may be initially excited from the synchronous condenser. To this end the direct current generator 15 will be operated as a shunt motor with energy" received from the direct current mains 24. The synchronous condenser then operates as a synchronous generator and is driven at substantially its synchronous speed of 3600 rpm. by the direct current machine 15. Alterna ing current of substantially 60 cycles is thus generated by the synchronous generator and supplied to the stator winding 13 of the induction generator, thereby furnishing the necessary initial exciting current for the latter machine.

In the equipment just described the pro peller-driving synchronous motor has 80 poles in order to obtain the low propeller speed with the alternating current energy of high periodicity (60 cycles) furnished by the high speed turbo-generator. While an 80 pole synchronous motor is not as unsatisfactory as an 80-pole induction motor, it i is, nevertheless, not an attractive design.

even in such a large capacity as some 10,000 KW. Two 40-pole motors eachof one-half the capacity of the 80-pole motor would be decidedly cheaper, would have better properties, and would go ina space which it would be more convenient to provide, owing to the smaller diameter and greater length. It is practicable to substitu'te' two machines each with 40. poles as the equivalent-of the 80-pole synchronous motor by employing the iarrangement diagrammatically illustrated in" Fig. 2. In this figure two component motor units A and B have their rotors mounted on the same pro-' peller shaft 33. The stator winding 36 of the motdr unit Ais a 40 pole three phase winding ada ted to be connected by means of a switch 3 to the bus bars 21 of the system of Fig. 1. The rotor of the motor .unit. A carries a -pole three phase winding 38 motor windings are connected as represented in Fig. 2 the combined motor units A and B will have a speed corresponding to an equivalent 80-pole motor, and hence the pro peller shaft 33 willbe driven at 90 rpm. By suflicient excitation of the continuous current fieldwinding 43 of the synchronous motor unit B, the current may lead sufliciently to provide the excitation for the induction generator of the system.

In Fig. 3 of the drawings there is diagrammatically illustrated a modified connection'of the windings of the motor units A and B which has the advantage of avoiding the need for any collector rings, or moving contacts of any kind. The stator winding 36 of the motor unit A of Fig. 3 is the same as the corresponding stator winding 36 in Fig. 2. The rotor winding 38 of the motor unit A of Fig. 3 is electrically win the shaft 33'. thus corresponds to the stator Winding-42 of connected to a corresponding three phase winding 42 on therotor'of the motor unit B, as for example, through an axial hole The rotor winding 42 the motor combination of Fig. 2. The stator winding 43 of the motor unit B is a Y- connected three phase winding whose terminals may be connected] by a switch to an external resistance 51 or to the D. C. mains 24. For conditions of navigation requiring relatively high motor torque, the switch 50 is thrown to connect the stator winding 43 to the external'resistance 51, as indicated in the figure. -The motor-units A. and B are then electrically connected as' two'concatenated induction motors, and due to the presence of the. external resistance 51 this concatenated induction motor combinationpossesses' highstarting torque. For normal conditions of navigation the switch, 50 .is thrown to connect t estator-winding 43 to 4 the D. C.mains24.= Assuming that the moto unit A has 40 poles and the motor unit nous motorco'n' bination can be operated with B 40 poles, then the speed of the propellerdriving motorcombination will correspond to a singleimachine having poles. Moreover, this "concatenated induction-synchro-.

I junity power. factor, or even with a leading current if the direct current field strength quently,

e made to '3 and 4..

carries a primary winding which is adapted -;the slot and the top-bar through the the drawings. This deep bar doub is raised by a snfficient amount. Consethe concatenated motor combination of Fig. 3 furnishes a much more desirable load on any system than would a ropeller-driving induction motor, and, furt er more, it is free from the disadvantage of a straight synchronous motor drive in that it does not have any sliding contacts whatever.

In Fig. 40f the drawings there is diagrammatically represented a slightl modied combination of induction-sync ronous motor drive. ,The motor units and their windings of Fig. 4 are identical with the motor units and windings of Fig.2 and corresponding elements of the two figures. have been deslgnated by the same reference nu- ,merals. In the combination .of Fig. 4 a

double-throw switch 60 is'provide'd for con- 1 necting the rotor winding 38 of the motor unit A to an external resistance-.61 or to the stator winding 42 of the motor unit B, Thus for navigating conditions requiring high motor torque the .switch 60 will be thrown to the position indicated in Fig. 4-

' and the propeller shaft 33 will be driven by the motor unit A which acts as an ordinary induction motor with a polar secondary winding connected to an external resistance.

For normal navigation the switch be thrown to its right-hand position, as

60 will viewed. in Fig. 4, thus giving the connections represented inaFig. 2 of In Fig. 5 of the drawings, the propellerdriving motor combination comprises an induction motor un t A and a synchronous motor. unit B, more orless analoous to the corres ipnding motor units 'of lgs. 2, he stator of the motor unit to be connected to produceprimary mag- .netic fields of two different pole numbers.

As is well known in the art a single primary winding may be rovided for this purpose, such for examp e, as described in Alexanderson Patent No. 841,609, ,dated January 15, 1907. For the pur oses of ex planationj however, I have diagrammatiand 63 adapte tobe connected windings 62 64 to the bus bars 21, and such by a switch the drawings.

,cally represented two independent primary' independent windings are often preferable. 1,:

By way, of example, I will assume that the winding 62 is a 36-po1e winding, while the winding 63 is a 24-pole winding. The rotor of the motor unit A carries a 36-pole short circuited 65 is a ouble-layerwinding carried in deep slots 66 in the. magnetic core of the The two conductor bars per slot of rotor.

the short-circuited winding 65 are ofls'et.

olarwinding 65. The winding near their centers so that each conductor-" baris the 'bottom bar through one-half-of half of the slot as indicated in Fig.1

eayer other f- 7 of 'I he polyphase of Fig. 5 will,

short circuited winding is described in the U. S. patent to Kierstead, No. 1,188,182

dated June 20, 1916, and provides a short-.

are of relatively high frequency, that is,-

when the slip of the motor is large. In addition the rotor of the motor unit A" carries a 24-pole three phase winding 70 which is connected by conductors extending through the hollow shaft 33' of the motor combination to a 36-pole three phase rotor winding 71 of the motor unit B". The stator winding of the motor unit B" is a SG-pole winding. 72 adapted to be supplied with direct current from the mains 2 1. rotor winding 70 of the motor unit A is arranged in shallow slots 73 alternating with the deep slots 66 in the magnetic core of the rotor member of this motor unit.

The operation of the motor combination it is believed,-be understood from the foregoing description. For conditionsrequiring a high motor torque, such as for reversal after full s eed ahead or for maneuvering in' genera, the 36-pole primary winding 62 will be connected to the bus bars 21. The 24-pole rotor, windin 70 will be utterly inert under such cond1- tions, and the secondary current of the motor will flow entirely in the short circuited winding 65, and due to the design for driving of this winding the motor unit A" will possess a high starting torque] Conse quently', with this connection nothing hap' pens in the component motor B", the torque vided entirely in the motor component A". When the need for high torque has ceased the 24- ole primar winding of the motor unit A is connecte to the bus bars 21. Un-

der this condition of operation the shortcirresponding. motor-units of Figs. 2, 3 and-4,

cuited rotor winding 65 becomes ,inert since it is uninfluenced by the 24-pole winding 63. The shallow-slot polyphasejwinding 70, however, responds to the influence of the 24-pole stator winding 63. Due the connection of the polyphase rotor windings 70 and 71 the comound motor runs with a speed correspon ing to .thaenillilllheitrig4 oles on motor un1t B" on A", that is it runs at a speed correspondin to a '6Q-pole motor. A ,60-pole motor-w en supplied with alternating current energy of -cycles runs at 120 rpm.,

consequently the contatenated' inductlons chronous motor combination- A"--B",

rives the propeller at a speed of approximately 120 vrpm. By incrasin Y the continuous current excitation on t r 30-pole field winding 72 on the stator of t 1e motor unit B, the alternating current of the system can ultimately be made to lead which will relieve the field of the turbo-generator 'cycles. .Accor ingly, high speed the propeller shaft being pro-- from requiring so much excitation or heat in as would be required by a' simple 60- po e squirrel cage induction motor.

The concatenated induction-synchronous I duction-synchronous motor combination a very considerable art of the necessary excitation for a sync ronous generatorcan be supplied by the concatenated motor combi-" nation by over-exciting the sync'hronousmotor unit thereof. If a synchronous turbogenerator is thus employed, its field will require only about one-half as much copper when used to supply a compound concatenated motor of the type herein described, as when used to supply. a single equivalent squirrel cage inductlon motor. This will improve the design of the turbo-generator much more than would be realized at first sight, since the limit of design for a synchronous turbo-generator of 3600 rpm. is in the field heating. With a decrease of 50 per cent. in the amount of field copper and in watts required for excitation, successful designs for 6000 or 8000KW, 3600' r m, 60 cycle synchronous generators can be made as readily as is ordinarily the case with 3000 or 4000 KVA nerators for 3600 blpolar greater synchronous turbo-' nerators of ca ities than have eretofore been practi ca may'be designed for use in combination with my improved concatenated inducti'onsynchronous motor.

The motor unit A", and generally the cor- 'dividual numbers of poles. Thus, if the second unit has only one-quarter as many generator in my present system The precise rpm., 60

poles as the first unit, then three-quartersgof v the electricity received by the first unityis transformed therein into mechanical energy and only one-quarter is handed onas electricity to the second motor unit. The motor unit A consequently. has to be of'considerably larger size than if it only dealt with one-half the power. However, since it is,

of scarcely more than one-half the diameter of a 60-pole. induction motor, and since roughly'speaking the cost decreases as the cube of the diameter and only increases, roughly, as the square root'of, the length, the concatenated inductionsynchronous moiao , cage induction ,motor,

* tor combination of my present invention is Furthermore, my improved motor -combination has the great advantage of operating at unity power factor, or if desired with a leading power factor. A 1

What I claim as new and desire to secure by Letters Patent of the United States, is

1.- An electric system of ship propulsion a relatively 1 low synchronous mopropeller, a relacomprising a propeller, speed alternating current tor arranged to drive said 'tively high speed alternating currentgenera- '5 comprising a propeller,

bei

the induction propeller with high torquefor maneuvering tor for delivering alternatlng current energy to said synchronous motor, means for furnishing excitation to both said generator and said motor, an elastic fluid turbine coupled to said generator and a motor element adapted to produce a high torque for driving the propeller during conditions of navigation requiring high motor torques.

2. An eleetricsystem of ship propulsion oomprisin a propeller, an alternating current sync ronous motor arranged to drive said propeller, a bi-polar induction generator for-delivering alternating current energy to said synchronous motor, means for furnishing excit-ation'to both said generator and said motor, a high speed elastic fluid turbine directly coupled to said generator, the synchrono'us speed of said synchronous motor elastic fluid turbine and an induction motor element supplied with energy from the induction generator for operating the propeller with high torque for maneuveringpurposes.

3. An electric system of ship propulsioncomprising a propeller, a relatively low speed alternating current synchronous motor arrangedto drive said propeller, a bi- H polar induction generator for deliverin alternating current energy to said synclironous motor, a high-speed elastic :fluid turbine coupled to said generator, a high speed synchronous condenser electrically connected to saidgenerator and adapted to furnish all or a part of the excitation for the generator,

a relatively low speed direct coupling it to said synchronous condenser and adapted to'supply'the excitation for said synchronousmotor; and an induction motor element supplied 'withener from generator for operating, the

4:. An electric system of rent synchronous motor arrangedto drive said propeller,; an induction generator for delivering-alternating current energy synchronous condenser an duction tor and condenser and ng less than ofne-tenth the speed ,of-the rel erator whereby "inductionsync corres onds to thespted of a single motor-1 I havinga number ofpoles equal to the sum current generator having a speed reduction gearing for trically Ship xp'rb u li tn an alternating curto said synchronous motor, elastic flusi turbine cou led to said generator, a synchronous con enser electrically connected to said generator and adapted to furnish the excitation for the generator, an auxiliaiy direct current generator mechanicall coupled to said d ada ted to furnish the excitation for said synchronous motor and condenser and an induction motor element supplied with energy from the ingenerator for operating the propeller with high torque for maneuvering purposes.

5. -A system for electrically propelling Iships comprising a high speed steam tur-' me, generator direct-connected'to said turbine, a propeller, a low speed polyphase synchro-- nous motor direct-connected to sald propeller and adapted to be supplied with poly phase electric energy from said generator, a high speed poly hase synchronous condenser for supplying all or a part of the excitation of said induction generator, a, continuous current enerator, a speed reduction gearing for coup ing said continuous current generator to the synchronous condenser to be driven thereby, electrical connections whereby said continuous current generator supplies the excitation for said synchronous moan induction motor element supplied with energy from said generator for operating the propeller with high torque for maneuvering purposes. I

6. An electric system of ship propulsion comprising a high speed elastic fluidturbine,

a squirrel cage polyphase induction a bi-polar induction generator of the squirtype directly coupled to said turbine, a ropeller shaft, an induction motor unit an a synchronous motor unit hav i? their rotors mounted on said propeller sha t,

cage

means for electrically connecting the induction motor unit to said generator for startmg and operating the propeller with high torque'for maneuvering purposes and means.

for electrically conne ting said motor units in-concatenation to said induction genhronoiujs motor combination of the poles ofsaidtrio motor units.

tor for ingand. (1191111 the "re elle1 with high P p r 7-v An electricisystem of shi p l- 1202' mg mountedon said 'propeller shaft and means-for electrically connect-I 'ing the. induction motor unit to said genera torque for maneuvering purposes.

the of the concatenated v I said two motor units for ordinary navigation. and means whereby the motor motor operationr v of ship propulsion v secondary rotor winding,

'rotor winding adapted nected to the secondary rotor windi 95 8. An electric system of shi propulsion comprising a propeller shaft, an induction motor unit having its rotor mounted on said shaft, a synchronous motor unit having its rotor also mounted on said shaft, an elastic fluid turbine, an alternating current generator direct connected to said turbine, and

means for electrically connecting said induction motor unit to said generator for induction motor operation for conditions of navigation requiring a large motor torque and for electricall concatenating said two motor units for ordinary navigation of the ship.

9. An electric system of ship propulsion comprising an elastic fiuidturbme, an alternating current generator coupled to said turbine, a propeller shaft, an induction motor unit having a primary-stator" winding electrically connected to said generator and a and a synchronous motor unit having a direct current stator winding and an alternating current rotor winding electrically connected to the secondary rotor winding of said induction-motor unit for ordinary navigation of the ship,

the rotors of said two motor units being,

mounted on said propeller shaft, and means .for operating the induction motor unit alone for conditions of navigation requiring a large motor torque.

10. An electric system of ship propulsion comprising a propeller shaft, an induction motor unit having a primary stator winding and a secondary rotor winding, a synchronous motor unit having a direct current stator winding and an alternating current to be electrically consaid induction motor unit, an elastic turbine, an alternating current generator direct connectedto said generator, means for electrically connecting the primary of said. induction motor unit to sa d genera- .tor for induction motor operation for conditions of navigation requlring a large motor torque for electrically concatenating of the ship, torque is accentuated during the induction 11. :An electric system comprising, an

winding;

comprising a motor unit having a primary stator winding adapted to'be connected to roduce primary magnetic poles of two di erent pole numbers and two secondary rotor windings, one ofsaid secondary -windings being a locally shortcircuited winding responsive to the higher pole number of said primary winding and the other of said secondary windings being a polar winding responsive lower pole number said shortrcircuited secondary winding having'relatively high resistance when the secondary current 18 of relatively high frequency, asynchronous motor unit having a direct current stator winding and an alternating current rotor winding electrically connected to the polar secondary winding of said induction motor unit, a source of alternating current ener and means for electrically connecting the primary winding of said induction motor unit to said source so as to produce'its higher pole number for motor torque and mary winding to said source so as to produce its lower pole number for ordinary navigation of the ship, y

12. An electric system of ship propulsion elastic fluid turbine, an, induction generator coupled thereto, a proeller shaft, an induction motor unit having its rotor mounted electrically connecting the induction motorunit to said generator for conditions of naviglziition requiringlar e motor torque, a sync ronous motor unit aving its rotor mounted on said shaft, means for electrically concatenati the induction and synchronous motor units for ordinary navigation of the ship, a synchronous condenser electrically connected to furnish excitation to said induction generator, erator mechanically coupled to said synchronous condenser and electrically connected to furnish excitation to said synchronous HENRY- M. HOBART.

propeller shaft, an induction to the of said primary winding,

cal

on said shaft, means for and a direct current gen- 9 

